You can use either iTunes or Logic for this.
1) With Logic open up a new project. Then import the audio file on your Desktop into the Logic project. You can do this by simply dragging the file from your Desktop into Logic.
Then go to File > Bounce.
Under [b]Destination[/b] make sure that MP3 is ticked. Then change the [b]Bit Rate Stereo[/b] to 320kbps. This is the highest resolution MP3 file that Logic can export. Then click [b]Bounce[/b] and choose where you wish to bounce the file.
2) In iTunes drag the audio file into a playlist.
Then go into iTunes > [b]Preferences[/b]. Under [b]General[/b] choose [b]Import Settings[/b]. Choose MP3 Encoder in [b]Import Using:[/b]
Under setting choose Custom and navigate down to 320kbps for the Stereo Bit Rate.
Then go back to your playlist. Right the track and choose [b]Create MP3 version[/b].
This will save an MP3 version in your Users > username > Music > iTunes > iTunes Media folder.

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